1. Field of the Invention
This invention relates to data transformation and transmission, and in particular to a device for compressing data from a fast access storage device and for transmitting compressed data to a slower medium.
2. Description of the Prior Art
Compression or transformation of data usually results in an irregular flow of compressed data which is not conducive to efficient recording on magnetic tape or transmitting over communication channels with reasonable efficiency. Computers with large, volatile direct access main storage devices usually require that the data contained therein be saved on nonvolatile, removable media such as tape for archival and backup purposes. The amount of data to be stored in conjunction with the relatively slow sequential access speed of tape storage devices compared to the fast access speed of direct access storage devices has led to significant efforts to compress data and to increase the speed of tape storage devices. However, there is still a disparity in their relative speeds which is usually dealt with by first storing blocks of data from the direct access storage device into a buffer which is dedicated to providing data to tape during save operations.
In U.S. Pat. No. 4,360,840 to Wolfrum et al, data is compressed in real time as it is produced by a facsimile raster scanner and stored in a buffer for transmission. Data is not transmitted until a full page of text has been compressed and stored. While this procedure reduces the amount of buffer space required to store a page of data, it does not address the problem of transmitting the data from the buffer while it is being compressed. This can result in loss of valuable transmission time and possibly require larger storage devices to store a full page of text.
U.S. Pat. No. 3,490,690 to Apple et al relates to compression of an instruction trace and recording the compressed data to tape. Compressed data is written to tape and sections of data are dropped and not recorded when the tape gets behind or "no data" characters are recorded on the tape when the tape gets ahead. This results in lost data and does not optimize the data storage capability of the tape.